Adding a ReactiveNavigator to handle toActivityForResult in an Observable way

The aim of this component is to handle the navigation to an Activity for result using an Observable and thus avoiding the need to check for result/response code and all the boilerplate code related.
:+1: no need for a custom operator here.

As a rule of thumb, if all you do is implement `onNext`, a map/flatMap is likely all you need. Custom operators are more meant to significantly change the way the data flows, such as buffering (e.g. batching up downstream onNext calls and then forwarding them in one swoop)